1. Field of the Invention
The present invention regards a rotation unit for a torque tong for making and/or breaking threaded connections between pipes and/or spinning pipes during screwing and/or unscrewing of pipes, primarily pipes used in petroleum production.
2. Description of Background Art
A prior art torque tong is described in NO 163973, which concerns a torque tong arranged both to break and make a threaded connection between two pipes, and also spin one of the pipes relative to the other in order to uncouple the pipes from each other or tighten the connection.
In older solutions, a special device was used to make and break the connection, while another special device was used to spin the pipes apart or together. The solution of NO 163973 allowed both making/breaking and spinning to be carried out in the same apparatus.
The solution of NO 163973 also entailed the advantage of being able to handle pipes within a wide range of diameters.
In order to achieve this, NO 163973 proposes the use of one or more master cylinders which upon rotation of the rotary part of the tong, and as a result of the placement of the cylinders, are pressed together, applying pressure to a number of slave cylinders. The slave cylinders will in turn displace jaws to engage one of the pipes involved, ensuring that these maintain a sufficiently powerful grip on the pipe to break or make the connection to a prescribed torque without the jaws slipping relative to the pipe.
A solution similar to that of NO 163973 has been described in NO 306572. Here the jaws are also equipped with respective slave cylinders. These are pressurized by a master cylinder mounted on the rotary part, which master cylinder is then influenced by a piston mounted outside the rotary part. The jaws are brought into engagement with the pipe by increasing the pressure from the master cylinder Valves ensure that the pressure in the slave cylinders is maintained independently of the master cylinder.
A considerable disadvantage of the latter of the above solutions is that the jaw travel is restricted by the displacement of the master cylinder. In a subsequent patent application (WO 00/45027) from the same applicant as NO 306572, it is stated that in the solution of the latter patent, the piston must push the master cylinder repeatedly in order to provide a sufficient volume of hydraulic fluid to push the jaws into engagement and also achieve sufficient retaining power. This causes a significant delay in the operation. In WO 00/45027, this problem is apparently solved by means of pressure accumulators.